Probes called "oxygen sensors" are commonly used to measure the oxygen content of gases in a heat treating furnace. Blumenthal U.S. Pat. No. 4,588,493, entitled "Hot Gas Measuring Probe," describes probes that can be used for this purpose.
The probe is typically installed in the heat treating furnace in direct contact with the hot atmosphere used for heat treating. The probe includes a solid electrolyte. One side of the electrolyte contacts the hot furnace atmosphere to be measured. The other side of the electrolyte contacts a reference gas, whose oxygen content is known. A voltage (measured in millivolts) is generated between the two sides of the electrolyte.
The magnitude of this voltage E(mv) is related to temperature and the difference between the oxygen content in the measured atmosphere and the oxygen content in the reference gas, as expressed in the following formula: ##EQU1##
Since the oxygen content of the reference gas [P.sub.O2 (Ref)] is known, one can therefore determine the oxygen content of the furnace atmosphere [P.sub.O2 ] by measuring the probe voltage [E(mv)] and the temperature T(.degree.K).
The probe voltage is usually measured by an associated controller outside the furnace. The controller compares the measured voltage to a "set point" voltage. The controller drives valves to alter the mixture of gases forming the atmosphere to maintain the desired oxygen content within the furnace.
The probe typically has associated with it a thermocouple. The thermocouple is located within the furnace to measure the temperature of the heat treating atmosphere. The thermocouple generates a voltage (also measured in millivolts) that represents the temperature conditions within the furnace.
This voltage signal representing the temperature conditions measured by the thermocouple may also be processed by the controller. By using the measured temperature and probe voltages, the controller generates a process variable (PV) expressing conditions within the furnace directly in percent oxygen (O.sub.2), dew point, or in percent carbon.
Maintaining the desired oxygen content in a carrier gas at some specific temperature within the furnace controls the heat treating atmosphere. Metals Handbook, Vol. 4, pp. 417-431 (9th Edition 1981) contains a further discussion of atmosphere control in a heat treating furnace.
Accurate control of carbon potential in the heat treating industry requires accurate input from both the thermocouples and the oxygen sensor probes. When a probe or thermocouple unexpectedly fails, or if its performance declines during use, the results can be economically catastrophic. Without accurately functioning sensor probes, the carbon potential within the furnace can no longer be reliably controlled. The heat treating process may have to be suspended while the faulty probe or thermocouple is replaced. The load of metal parts undergoing heat treatment within the furnace at the time of failure also may have to be scrapped or reworked. If the metal parts are expensive (for example, the landing gear of an airplane), the economic loss can be tremendous.
Even without massive probe failure, the accuracy of a probe can be adversely affected by gradual degradation in performance over time. There are also process-related problems adversely affecting probe accuracy. Soot can build up on the probe exposed to the harsh atmosphere in the furnace. Soot buildup degrades the performance and the accuracy of the probe. Chemical contaminants can also coat or deteriorate the electrolyte surface and cause inaccurate probe readings.
The accuracy of a thermocouple can be manually checked by comparing its output with a thermocouple traceable to the National Institute of Standards and Technology, following ASTM 2750. See Blumenthal et al, "Check Out Carbon Control System Step by Step," Heat Treating, August 1991. This manual procedure is well established and is followed in the heat treating industry.
However, the procedure for periodically checking the accuracy of oxygen sensor probes is not as well understood and established. One suggested way is to perform a weight-gain measurement of an equilibrated steel shim. See, e.g., Blumenthal el al., Ibid. This steel shim procedure, like the thermocouple procedure, is done manually and can be laborious.
There are other probe test methods that check specific components of the probe, like the outer electrode, but not the overall performance or accuracy of the probe. These specific test methods can lead to a false sense of security. Component-specific tests may overlook or fail to detect degradation in probe performance or accuracy caused by other probe components that are not checked.
The different, more subtle failure modes of today's high quality oxygen probes are not widely appreciated. The onset of operation that foretells future probe failure often goes undetected, because it is illusive to detect using today's methodologies. As a result, probe failure, or the accumulation of soot, can occur without apparent warning.
Despite all reasonable precautions, conventional furnace control systems remain subject to costly process disruption due to the sudden failure or undetected gradual decline of performance and accuracy of probes and thermocouples.